1. Field of the Invention
The invention relates to a low pass filter for the removal of harmonics, which is connected to an output stage of a D-class amplifier having a plurality of channels.
2. Description of Related Art
In recent years, amplifiers using the so called D-class amplification have been widely used following the demand for miniaturization and realization of a high efficiency amplifying apparatus.
The D-class amplification is an amplifying scheme in which a modulating process such as pulse width modulation (PWM) or pulse density modulation (PDM) is performed to an analog signal supplied to an amplifying circuit, the processed analog signal is converted into a digital signal, thereafter, the digital signal is amplified, harmonics in an output are removed by a low pass filter, and the digital signal is returned to the analog signal. According to a D-class amplifier, since the amplification of the digital signal, that is, the amplification of the analog signal can be performed only by an ON/OFF process of the signal, logically electric power efficiency of 100% can be obtained.
As a low pass filter for an output stage of the D-class amplifier, it is general to use a low pass filter of the so called LC type which comprises an inductor and a capacitor and of which an electric power loss is small. For use in the D-class amplifier, it is also desirable to miniaturize the low pass filter used at the output stage so that circuit elements arrangement on a printed circuit board can be integrated. This is because the high electric power efficiency and the miniaturization can be accomplished with a D-class amplifier, as mentioned above.
Since coils obtained by winding conductors are generally used as inductors which are used for the low pass filter, however, if a load current flows in the inductors included in the low pass filter provided for one channel output, a change in magnetic flux occurs. When the magnetic flux is interlinked with inductors included in a low pass filter of another channel, an induced electromotive force is caused in the other inductor due to an electromagnetic induction phenomenon.
Therefore, when the low pass filters at the output stage of the D-class amplifier having a plurality of output channels are integrated on a printed circuit board and arranged closely, interlinkage of the leakage magnetic flux from the inductor or inductors in the low pass filter of the adjacent channel may occur in the inductor or inductors of the low pass filter of the other channel. In that case, the leakage of the signal occurs between channels, and the separation between channels eventually deteriorates.
In other words, in the case of the multichannel D-class amplifier, it is extremely difficult to preferably maintain separation between both of the adjacent channels while realizing the miniaturization and integration of the low pass filter at the output stage.
In order to improve the channel separation, it is conceivable to use an inductor having a core of a closed magnetic path type in which a leakage magnetic flux is small. With respect to the output stage of the D-class amplifier, it is however necessary to provide a magnetic gap for a magnetic path so that it is difficult to completely eliminate the leakage magnetic flux because the load current at the output stage is usually large and magnetic saturation is likely to occur.